It is common practice in modern medical treatment to perform treatment and testing of various kinds by using a medical instrument called a catheter which is a long thin tubular body. There are several methods for treating through use of a catheter as follows: administering drugs directly to the lesion owing to the length of the catheter; expanding a stenosis in the lumen with the help of a catheter having a balloon capable of inflation under pressure which is attached to the distal end of the catheter; opening a stenosis with the help of a catheter having a cutter attached at its distal end to shave off the lesion; and closing an aneurysm, nutritive blood vessel, bleeding part, etc. with a packing delivered by a catheter. Another treatment method involves placing a stent in the lumen with the help of a catheter to keep stenosis open with the help of a catheter. A sent is a cylindrical body whose side wall is of netlike structure. Further, extraction of excess fluid from the living body with the help of a catheter is another example of use.
The treatment and testing with a catheter are usually accomplished by percutaneously inserting a catheter into a blood vessel having a lesion through the lumen of an introducer sheath which is attached to a catheter introducer and punctured into the patient's arm or leg.
Japanese Patent Laid-open No. Hei 08-168532 discloses an introducer assembly which has an introducer sheath and a dilator which are previously integrated with each other so as to reduce work associated with assembling on the site of treatment and examination. The disadvantage of previously integrating an introducer sheath and a dilator with each other is that the resulting assembly in which the dilator tube passes through the hemostatic valve attached to the introducer sheath remains in such a state until it is actually used. The dilator tube passing through the hemostatic valve exerts a compressive force on the hemostatic valve for a rather long period of time. As a result, the hemostatic valve undergoes permanent set after a lapse in time. The resulting hemostatic valve would not completely arrest the flow of blood after the dilator tube has been pulled out. In the case of the introducer assembly disclosed in Japanese Patent Laid-open No. Hei 08-168532, this problem is addressed by forming the dilator tube in such a way that the part in its lengthwise direction which remains in contact with the hemostatic valve is smaller in outside diameter than any other parts. This structure protects the hemostatic valve from the compressive force acting on it for a long period of time, thereby allowing the hemostatic valve to fully function after the dilator tube has been pulled out.
However, making the dilator tube partly smaller in outside diameter suffers the disadvantage that the dilator tube receives a concentrated stress at the part where the diameter changes and hence tends to easily break. As a result, the dilator tube does not fully function as the core of the sheath tube which is the inherent function of the dilator tube.